Gamma Camera Dead Time Determination in Real Time Using Long Lived Radioisotopes

ABSTRACT

For dead time determination for a gamma camera ( 18 ) or other detector, a long-lived point source ( 26 ) of emissions is positioned so that the gamma camera ( 18 ) detects ( 34 ) the emissions from the source ( 26 ) while also being used to detect ( 36 ) emissions from the patient ( 22 ). The long-lived point source ( 26 ), in the scan time, acts as a fixed frequency source ( 26 ) of emissions, allowing for dead time correction measurements that include the crystal detector effects.

RELATED APPLICATIONS

The present patent document claims the benefit of the filing date under35 U.S.C. §119(e) of Provisional U.S. Patent Application Ser. No.62/008,791, filed Jun. 6, 2014, which is hereby incorporated byreference.

BACKGROUND

The present embodiments relate to single photon emission computedtomography (SPECT). In particular, the present embodiments relate todead time correction in SPECT.

During SPECT imaging, the detector electronics take time to performdetection of an emission. During this period, additional emissions arenot detected due to the unavailability of the detector electronics. As aresult, the actual emissions may be under counted. The count of detectedemissions is corrected for dead time. In one approach, a signal is inputat fixed frequency and amplitude to the detector electronics. Due to thefixed frequency, a known number of signals is input. Due to dead timefrom detecting emissions from the patient, some of the fixed frequencysignals are not detected. The ratio of detected ones of the fixedfrequency to the input number provides a measure of dead time. The countof detected emissions from the patient is divided by the ratio tocorrect for the dead time.

Since the fixed frequency signal is input at the electronics, anycontribution of the detector to the dead time is ignored. Thiscontributes to uncertainty in the reconstructed image. In quantitativeSPECT, an inexact correction may result in an inexact quantification.When imaging therapy isotopes with corresponding high-count rates, theinaccuracy may be more significant.

BRIEF SUMMARY

By way of introduction, the preferred embodiments described belowinclude methods, systems, and non-transitory computer readable media fordead time determination for a gamma camera or other detector. Along-lived point source of emissions is positioned at a fixed locationso that the gamma camera detects the emissions from the source whilealso being used to detect emissions from the patient. The long-livedpoint source, in the scan time, acts as a fixed frequency source ofemissions, allowing for dead time correction measurements that includethe crystal detector effects.

In a first aspect, a method is provided for dead time determination fora gamma camera. The gamma camera detects a count rate from aradioisotope source connected adjacent to the gamma camera and detectsemissions from within a patient while detecting the count rate. The deadtime is determined from the count rate. A count of the emissions iscalculated as a function of the dead time.

In a second aspect, a SPECT system includes a shielded point sourceconnected to emit radiation at a gamma camera. Detection electronics areconfigured to detect emissions, including the radiation from theshielded point source and radioisotope emissions from a patient. Aprocessor is configured to correct for dead time of the detectionelectronics, the correction being a function of real-time detection ofthe radiation from the shielded point source.

In a third aspect, a method is provided for dead time determination foran emission detector. A detector detects first emissions from a patientand second emissions from a point source. The second emissions aresubjected to dead time from the detection of the first emissions. Aprocessor corrects a count of the first emissions as a function of acount of the second emissions.

The present invention is defined by the following claims, and nothing inthis section should be taken as a limitation on those claims. Furtheraspects and advantages of the invention are discussed below inconjunction with the preferred embodiments and may be later claimedindependently or in combination.

BRIEF DESCRIPTION OF THE DRAWINGS

The components and the figures are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the invention.Moreover, in the figures, like reference numerals designatecorresponding parts throughout the different views.

FIG. 1 is a block diagram of a SPECT system, according to oneembodiment, with dead time correction;

FIG. 2 is a cross-sectional side view of one embodiment of a detectorand collimator with an added long-lived point source; and

FIG. 3 is a flow chart diagram of one embodiment of a method for deadtime determination for an emission detector.

DETAILED DESCRIPTION OF THE DRAWINGS AND PRESENTLY PREFERRED EMBODIMENTS

A long-lived radioisotope is used for absolute system dead timedetermination in real time for a gamma camera. The detectors are exposedto the long-lived radioisotope at the time of patient acquisitions.Emissions from the long-lived radioisotope are regular and separablebecause of fixed spatial location and distinct emission energy fromemissions in the patient by the radiotracer being imaged. The dead timeis measured using the long-lived radioisotope, and the count ofemissions from the radiotracer is corrected for the dead time. Real timemeasurement of the system dead time is incorporated as part of the inputdata from a patient scan, measuring the true system dead time at thetime of the acquisition.

FIG. 1 shows one embodiment of a single photon emission computedtomography (SPECT) system 10 for dead time correction. The system 10includes a processor 12, a memory 14, a display 16, a detector 18,detector electronics 20, and a shielded source 26. The processor 12,memory 14, and/or display 16 are part of the SPECT system 10 or areseparate (e.g., a computer or workstation). Additional, different, orfewer components may be provided. For example, user input, patient bed,or other SPECT related devices are provided. Other parts of the systemmay include power supplies, communications systems, and user interfacesystems. Any now known or later developed SPECT system 10 may be used.As another example, the display 16 is not provided.

The detector 18 is a gamma camera connected with a gantry. The gammacamera may include the detector circuits 20 and the detector 18, or justthe detector 18. The gamma camera is a planar photon detector, such ashaving crystals or scintillators with photomultiplier tubes or otheroptical detector. The gantry rotates the gamma camera about the patient.During scanning of a patient, emission events are detected with thecamera at different positions or angles relative to the patient.

The detector 18 has any shape. For example, the detector 18 has a squareor rectangular detection surface in a plane orthogonal to the patient.Other shapes may be used.

Referring to FIG. 2, a collimator 24 is positioned in front of, adjacentto, or by the detector 18. The collimator 24 is part of or connected tothe detector 18. The collimator 24 includes lead, tungsten, or othermaterial that is impervious to or absorbs and attenuates gammaradiation. The collimator 24 includes holes or other structures thatpass gamma radiation from some directions (e.g., more orthogonal) andlimit radiation from directions at other angles.

A shielded point source 26 is positioned relative to the detector 18.The shielded point source 26 is shielded in lead, tungsten, or othermaterial preventing or limiting exposure to the patient. The shield mayhave a hole, window, or gap in shielding for allowing emissions of gammarays from the point source 26 to impinge upon the detector 18. Any sizepoint source 26 may be used, such as a 1 mm³ vessel with the long-livedradioisotope. Line or other shaped sources may be used.

The point source 26 is a long-lived, factory-calibrated point source.The radioisotope of the point source 26 is long lived relative to theradioisotope ingested by or used for emitting gamma rays from thepatient. If the half-life of the radioisotope is long enough (e.g., 6months, 1 year, or more), then the rate of decay from the radioisotopeis essentially constant during the patient acquisition for a givenpatient. The radioisotope of the point source 26 essentially acts as afixed frequency signal but one that interacts with the entire imagingchain including the detector 18 and not just the signal processingdetector circuit or electronics 20. Some example radioisotopes for thepoint source 26 include a single emission radioisotope, such as 182 Hfwith T½ of 8.9E6Y, E-Gamma of 270.4, and BR of 79.0, or a multi-emissionradioisotope, such as Ba133 with T1/2 of 10.55 y, E-Gamma of 81.0,276.4, and 302.9+356.0+383.8 keV, and corresponding BR of 32.9, 7.2, and89.3%, or 176 Lu with T½ of 3.76E10y, E-Gamma of 88.3, 201.8, and 306.8,and corresponding BR of 14.5, 78.0, and 93.6. Other radioisotopes may beused.

The shielded point source 26 is connected to emit radiation at thedetector 18 in a repeatable or known position. The connection is byweld, bolt, latch, press fit, threading, or other connection to thecollimator 24, detector 18, gantry, frame, or other structure. Theshielded point source 26 may be added to an existing detector 18 orcollimator 24, such as adding a bracket to attach the point source 26 toa frame holding the detector 18. The shielded point source 26 may bedesigned to fit in or be part of the collimator 24. For example, athreaded hole is formed in the collimator 26. The shield of the shieldedpoint source 26 includes matching threads for attaching.

The connection positions the shielded point source 26 so that the holeor window in the shielding is directed at the detector 18. Thepositioning angles the point source 26 to pass gamma emissions throughthe collimator 24 to the detector 18.

The point source 26 is positioned anywhere in front of the detector 18.In one embodiment, the point source 26 is positioned at a corner orother region that may not detect many gamma rays from the patient. Dueto the collimation, the edge or corner of the detector 18 may be lesslikely to detect emissions from the patient. As a result, the shieldedpoint source 26 is less likely to interfere with detection of emissionsfrom the patient 22. Due to size, the point source 26 exposes or coversa small part (e.g., less than 1%) of the detector 18. The point source26 is placed against or in the collimator 24 or is spaced from thecollimator 24.

The detector electronics 20 include pulse arithmetic circuits, pulseheight analyzer, digitizer, filter, analog-to-digital converter,application specific integrated circuit, field programmable gate array,signal processor, combinations thereof, processor 12, or other now knownor later developed circuit for detecting the position and energy of eachemission on the detector 18. A processor may be provided for pile-uphandling. The detector electronics 20 receive the output of thephotomultiplier tubes or other light detector of the detector 18 andoutput a position, time, and energy level. The detector electronics 20may include a threshold function, filter, or other process for rejectingemissions due to unresolvable pile-up or energy not in an expectedwindow or range for the radioisotope.

The detector electronics 20 detect emissions including the radiationfrom the shielded point source 26 and radioisotope emissions from apatient 22. Using a radioisotope marker (i.e., point source 26) for deadtime determination during the patient acquisition does not require anymodifications to detector electronics 20. The detector electronics 20may apply a different energy range filter to distinguish betweenemissions from the point source 26 and the patient 22. For example, theradioisotope for the patient is Tc-99m with peak energy of emissions at140 keV, and the point source 26 uses 182 Hf with peak energy ofemissions at 270.4 keV. By detecting the energy as being within 10% orother range of 140 keV, emissions from the patient are detected. Bydetecting the energy as being within 10% or other range of 270.4 keV,emissions from the point source 26 are detected. The detectorelectronics 20 or other processor counts the number of emissions for agiven energy range. The count is an absolute count or is a count rate(i.e., number of emissions per unit time). The energies for theradioisotope of the point source 26 are separable or distinguishablefrom the energies of the radioisotope used in the patient 22. Emissionswith energies outside the ranges are not counted or are discarded.

The detection by the detector 18 and detector electronics 20 occursduring a scanning session for a patient 22. The patient 22 is positionedwithin the gantry or on a bed of the SPECT system 10. For imaging uptakein a patient, the detector 18 detects emissions from the patient 22. Theemissions occur from any location in a finite source (i.e., the patient22). The radiotracer in the patient migrates to, connects with, orotherwise concentrates at specific types of tissue or locationsassociated with specific biochemical reactions. As a result, a greaternumber of emissions occur from locations of that type of tissue orreaction. For example, the radiotracer is designed to link withlocations of glucose uptake, fatty acid synthesis, or other metabolicprocess. A given imaging session occurs during one scanning appointmentand/or ingestion or injection of the radiotracer for a given instance ofSPECT imaging.

In one embodiment, the detector electronics 20 performs pile-upseparation. Emissions may occur rapidly enough in sequence that energyfrom one emission may result in a later emission appearing to havehigher energy. By separating out the emissions and accounting forpile-up, emissions at the desired energies may be more accuratelydetermined without discarding actual emissions that should bemaintained. Since emissions from different energies are used (e.g., fromthe patient 22 and the point source 26), pile-up processing may alterthe calculation of dead time. In one embodiment, the alteration isacceptable. In another embodiment, the pile-up processing is not used.Instead, the detector electronics 20 are operated without pile-upseparation. A fully integrated mode (i.e., detect based on energywithout attempting to account of energy tails from other emissions) isused.

The processor 12 is a general processor, digital signal processor,graphics processing unit, application specific integrated circuit, fieldprogrammable gate array, digital circuit, analog circuit, combinationsthereof, or other now known or later developed device for processingemission information. The processor 12 is a single device, a pluralityof devices, or a network. For more than one device, parallel orsequential division of processing may be used. Different devices makingup the processor 12 may perform different functions, such as oneprocessor (e.g., application specific integrated circuit or fieldprogrammable gate array) for reconstructing and another for correctingan emission count for dead time. In one embodiment, the processor 12 isa control processor or other processor of the SPECT system 10. In otherembodiments, the processor 12 is part of a separate workstation orcomputer.

The processor 12 operates pursuant to stored instructions to performvarious acts described herein, such as performing acts 38, 44 and 46 ofFIG. 3. The processor 12 is configured by software, firmware, and/orhardware to perform, control performance, and/or receive data resultingfrom any or all of the acts of FIG. 1.

In one embodiment, the processor 12 is configured to correct for deadtime of the detector electronics 20. During the detection processing,the detector electronics 20, of which the processor 12 may be part,cause a delay. Any emissions occurring during the delay are notprocessed or are not detected. This delay of nanoseconds or microsecondsis the dead time. To correct for the dead time, the processor 12determines a scaling factor representative of the percentage or numberof emissions that occur but are not detected due to dead time. The countof the detected emissions is increased based on the scaling factor.

In one embodiment, the processor 12 uses the real-time detection of theradiation from the shielded point source 26 for the correction. Thecount of the emissions from the shielded point source 26 is used. Sincethese emissions are detected while also performing operations to detectthe emissions from the patient with the same detector, the emissionsfrom the point source 26 are subject to the dead time of the detectorelectronics 20. The point source 26 generates emissions at a regular orknown rate, so the number of emissions during the count of emissionsfrom the patient is known.

Alternatively, the number of emissions from the point source 26 ismeasured at another time when there is no dead time, such as aftercalibrating the SPECT system 10 but without a phantom or patient (e.g.,after each of monthly calibrations). The emissions from the point source26 are measured when there are no emissions from a radiotracer in apatient. By measuring the count instead of using an assumed count,effects due to misalignment or other variables are more likely includedin the count. The assumed or measured count from the detector 18 with nodead time is stored in the memory 14 for use in dead time correction.

The processor 12 calculates a ratio of a number of point source 26emissions in a given time period during the scan of a patient to thenumber of emissions when there is no dead time. This ratio indicates thescale factor. The ratio indicates what percentage of actual emissions ismissed due to dead time so that the detected count may be increased toaccount for dead time. The ratio of the count rate from the point source26 during a patient acquisition to the count rate when there is no deadtime provides the scale factor. Since the ratio depends on a measurementduring patient scanning, the ratio is a real time measure of the systemdead time at the time of patient acquisition. In other embodiments, adifferent function than a ratio is used.

The processor 12 is configured to scale the count of the emissions fromthe patient 22. Any or all counts are scaled, such as counts for eachlocation on the detector. The count is scaled by the scaling factor. Theratio of counts from the point source 26 with and without dead time isused to scale the counts from the patient. The ratio of counts ofemissions from the patient with and without dead time is the same as theratio from the point source 26. The count from the patient without deadtime is unknown, so the count of emissions from the patient with deadtime is divided by the ratio of the emissions from the point source. Forother functions than ratio, multiplication or other functions foradjusting or increasing the count of the emissions from the patient areused. By weighting the count of the radiotracer emissions from thepatient based on the number of emissions of radiation from the pointsource 26, a more accurate count is provided.

For a given imaging session, a single correction is used. Alternatively,the ratio or scale factor is calculated for different periods, such asdifferent positions of the detector 18 relative to the patient. Thecounts for each of the periods are corrected based on weights measuredfor that respective period.

The SPECT system 10, using the processor 12 or another processor, isconfigured to reconstruct the imaged volume by applying a system matrixor forward projection to the corrected counts. The emissions from thepatient, as corrected for dead time, are used in reconstruction. Anyreconstruction may be used to estimate the activity concentration in thepatient. The SPECT system 10 accesses the detected emission events fromthe memory 14 or buffers to reconstruct. Based on the corrected countsfor the emission bins from different locations on the detector, theprocessor 12 is configured to calculate specific uptake values (SUVs) asa function of location in the patient. The SUV at one or more locationsare calculated by normalizing the activity concentrations as representedby the counts with a dose for the radioisotope in the patient 22.Alternatively, activity concentration without SUV is used in thereconstruction.

The detected emission events, other functional information, or otherscan data is stored in the memory 14. The data is stored in any format.The memory 14 is a buffer, cache, RAM, removable media, hard drive,magnetic, optical, database, or other now known or later developedmemory. The memory 14 is a single device or group of two or moredevices. The memory 14 is part of SPECT system 10 or a remoteworkstation or database, such as a PACS memory.

The memory 14 may store data at different stages of processing, such asa count and counting period from the point source 26 without dead time,count and period during patient scanning, raw data (e.g., energy andlocation) representing detected emissions from the patient withoutfurther processing, filtered or thresholded data prior toreconstruction, reconstructed data, filtered reconstruction data, asystem matrix, forward projection information, projection data,thresholds, an image to be displayed, an already displayed image, orother data. The memory 14 or a different memory stores the ratio orother scale factor for correcting for dead time. The memory 14 or adifferent memory stores the corrected counts of emissions from thepatient. For processing, the data bypasses the memory 14, is temporarilystored in the memory 14, or is loaded from the memory 14.

The memory 14 is additionally or alternatively a non-transitory computerreadable storage medium with processing instructions. The memory 14stores data representing instructions executable by the programmedprocessor 12. The instructions for implementing the processes, methodsand/or techniques discussed herein are provided on non-transitorycomputer-readable storage media or memories, such as a cache, buffer,RAM, removable media, hard drive or other computer readable storagemedia. Computer readable storage media include various types of volatileand nonvolatile storage media. The functions, acts or tasks illustratedin the figures or described herein are executed in response to one ormore sets of instructions stored in or on computer readable storagemedia. The functions, acts or tasks are independent of the particulartype of instructions set, storage media, processor or processingstrategy and may be performed by software, hardware, integratedcircuits, firmware, micro code and the like, operating alone or incombination. Likewise, processing strategies may includemultiprocessing, multitasking, parallel processing and the like. In oneembodiment, the instructions are stored on a removable media device forreading by local or remote systems. In other embodiments, theinstructions are stored in a remote location for transfer through acomputer network or over telephone lines. In yet other embodiments, theinstructions are stored within a given computer, CPU, GPU, or system.

The display 16 is a CRT, LCD, plasma screen, projector, printer, orother output device for showing an image or quantity. The display 16displays an image of the reconstructed patient volume, such as showingactivity concentration as a function of location. The uptake function(e.g., SUV) of the tissues of the patient may represented in the image.Multi-planar reconstruction, 3D rendering, or cross-section imaging maybe used to generate the image from the voxels of the reconstructedvolume. Alternatively or additionally, any quantities derived by theprocessor 12 may be displayed, such as SUVs and/or change in SUV. Otherquantities may be determined, such as average SUV or activityconcentration for a region, maximum SUV, peak SUV in a predeterminedunit volume, variance in activity concentration, or total SUV. The imagevalues or quantity is based on counts that have been corrected for deadtime in real-time using the point source 26.

FIG. 3 shows one embodiment of a method for dead time determination fora gamma camera or other emission detector. The dead time is determinedand used to correct emission counts in qualitative and/or quantitativeSPECT. Real-time measure of dead time is used to correct the number ofdetected emissions from the patient. The method is applied for a givenscan of a given patient.

The method is implemented by the system of FIG. 1, the arrangement ofFIG. 2, both, or other system and arrangement. A processor performs acts38-46. A gamma camera or detector and detector electronics perform acts30-36. A long-lived source is used for performing acts 30, 32, and 34. Aradiotracer is used to perform acts 32 and 36. Other devices ormaterials may be used or controlled to perform any of the various acts.

Additional, different, or fewer acts may be performed. For example, act30 is not performed where the number of emissions from the long-livedsource is assumed or simulated. As another example, acts 44 and/or 46are not provided. In other examples, acts related to positioning thepatient, configuring the SPECT scanner, and/or SPECT imaging areprovided. The acts are performed in the order shown or a differentorder.

In act 30, emissions from a long-lived source are detected. A gammacamera or other detector detects emissions from a shielded sourcepositioned by the detector. As part of calibration or other time inwhich emissions from other radioisotopes are not also being detected,emissions from the long-lived source are detected. A count over time orrate of emission is determined. The determination is made while thedetector and electronics are not subjected to dead time. The emissionsare measure to establish a base line count rate for the long-livedsource. In alternative embodiments, the count rate from the long-livedsource is assumed or simulated. The count rate is stored and laterloaded from memory.

In act 32, emissions are detected during a scan of a patient. During thescan, the gamma camera or other detector detects emissions from anysource. The emissions are from the long-lived source and from aradioisotope in the patient. The emissions from the long-lived sourceare detected at a corner or other position relative to the detector. Byplacing a shielded source to direct emissions to the detector, theemissions may be detected. The radioisotope in the patient is aninjected or ingested liquid tracer. Emissions from the differentradioisotopes are detected during the scan of the patient.

The emissions from both sources during the patient scan are subjected todead time. The long-lived source may generate emissions periodically,but with enough separation to avoid dead time in the detection. Theemissions from the radioisotope in the patient may be less regular andmay occur with variable amounts of separation in time. Due to theemissions from the patient, other emissions from the patient and/oremissions from the long-lived source may not be detected due to deadtime. Some emissions from both sources are missed by the detector anddetection electronics.

During the time in which a patient is scanned (i.e., while the patientis positioned for scanning), the emissions are detected in real-time. Asthe emissions from the patient and the long-lived source occur, at leastsome of the emissions are detected.

Energy distinguishes the emissions from the different sources. Thedetection electronics threshold or window energies within differentranges. One range is provided for detecting emissions from theradioisotope in the patient. Another range is provided for detectingemissions from the long-lived source. The energy ranges do not overlap,allowing distinguishing between the sources of the emissions. A countand/or count rate of the emissions is separately measured by thedetector and detection electronics for each source.

The gamma camera, such as the detection electronics of the gamma camera,operates in a fully integrated mode. For SPECT, the fully integratedmode avoids pile-up processing. Rather than separate out emissions withpile-up processing, emissions are either in or not in the energy window.If multiple emissions pile up, then one or more of the emissions may bedetected as having an energy outside the range or ranges of interest. Asa result, the emission is not counted without pile-up processing. Byoperating in the fully integrated mode, distinguishing between emissionsfrom sources at different energies is avoided. Alternatively, pile-upprocessing is provided.

Act 32 is represented in FIG. 3 as including acts 34 and 36. Additional,different, or fewer acts may be provided for detecting emissions duringthe patient scan. Acts 34 and 36 are performed in any order in anon-going or repeating manner.

In act 34, emissions from a long-lived source are detected. The sourceis positioned adjacent to the gamma camera. The gamma camera detects theemissions. By having a greater half-life by a factor of at least ten(e.g., half-life in months or years) than a radioisotope in the patient(e.g., half-life in hours or days), the resulting emissions may betreated as a fixed frequency signal over the patient scan.

The processor or detection electronics determine a count and/or countrate for the emissions from the long-lived source. The number ofemissions or number over a period is calculated.

In act 36, emissions from within the patient are detected. As theradioisotope in the patient decays, gamma radiation is emitted. Thegamma camera detects the emissions.

The detection occurs while detecting the emissions from the long-livedsource. The emissions may occur at a same time or different times. Eachdetected emission results in dead time. Any following or subsequentemissions occurring in the dead time are not detected. The detection ofthe emissions continues during the patient scan with some emissionsbeing missed.

In act 38, one or more counts of emissions from the patient arecorrected. A processor increases the count to account for emissions thatoccurred during the dead times. Since emissions during the dead timesare not detected, the correction instead relies on a count or count rateof emissions from the long-lived source while subjected to the same deadtimes. The count or count rate while not subjected to the any dead timemay also be used. The counts for each of different positions on thedetector are corrected.

Act 38 as represented in FIG. 3 includes acts 40 and 42. Additional,different, or fewer acts may be performed to correct based on thedetections of acts 30-36.

In act 40, a dead time is determined from the count or count rate fromthe long-lived source based on the detections of act 34. The dead timeis a ratio or percentage of time during which detections cannot occur ascompared to the total. In one embodiment, the ratio of the count orcount rate of act 34 (e.g., count from long-lived source subject to deadtime) to the count or count rate of act 30 (e.g., count from long-livedsource not subjected to dead time) is calculated. This ratio indicatesthe relative amount of dead time to overall time of scanning or countingemissions from the patient. The ratio relies, in part, on measures ordetection in real-time with detection of emissions from the patient.

In act 42, the count of the emissions detected in act 36 is corrected.The emissions detected in act 36 are subject to undercounting due to thedead time. To more accurately reflect the number of actual emissions,the count is increased. The ratio from the long-lived source isindicative of the amount of undercounting. By dividing the count by theratio, the processor corrects the count. The count is increased toaccount for emissions likely or possibly missed during the dead time.Other functions to increase the count by a scaling factor based on thedetection of act 34 may be used.

In act 44, the processor calculates the activity concentration. Thecorrected count is used to estimate the activity at a given location orregion in the patient. The activity concentration may be the correctedcount or a number of the emissions for a given location. The activityconcentration in a patient having received the liquid radiotracer isdetermined as part of reconstruction by the SPECT system. Afteringesting or injecting the radiotracer into the patient, the patient ispositioned relative to the detector and/or the detector is positionedrelative to the patient. Emissions from the radiotracer within thepatient are detected over time. To determine the locations within thepatient at which the emissions occurred, the detected emissions, ascorrected for dead time, are reconstructed into an object space.

For reconstruction, the activity concentration (e.g., quantitativeSPECT) is reconstructed using a system matrix or forward projection.Distribution of emissions in a volume or image data is reconstructedfrom the detected emissions. The quantity or amount of uptake for eachlocation (e.g., voxel) may be estimated as part of the reconstruction incomputed tomography. The SPECT system estimates the activityconcentration of an injected radiopharmaceutical or tracer for thedifferent locations. In quantitative SPECT, the goal is to estimate theactivity concentration in kBq/ml of the tracer (i.e., isotope) that wasinjected into and distributed within the patient.

The reconstruction is iterative and contains a model of the imagingformation physics as a pre-requisite of quantitative reconstruction. Theimage formation model includes the detected data (e.g., correctedcounts), the system matrix or forward projection, isotope properties(e.g., dose value), and/or biology. The system matrix or forwardprojection represents mechanical properties of system, but may includeother information (e.g., injection time and patient weight asrepresented by SUV).

Reconstruction includes a projection operator that is able to simulate agiven SPECT system or SPECT class. Any now known or later developedreconstruction methods may be used, such as based on Maximum LikelihoodExpectation Maximization (ML-EM), Ordered Subset ExpectationMaximization (OSEM), penalized weighted least squares (PWLS), Maximum APosteriori (MAP), multi-modal reconstruction, NNLS, or another approach.

Specific uptake values (SUVs) may be calculated. The activityconcentration represents the amount of uptake at each location. Thisamount of uptake is a measure of emitted radiation, so is not normalizedfor the radiation dose provided to the patient. As a result, comparinguptake from different times may not be useful unless the same dose isprovided. By calculating the SUV, uptake normalized for dose isprovided, allowing comparison of different measures.

In act 46, a SPECT image is generated. The corrected count is used inthe reconstruction. Where quantitative SPECT is not provided, thecorrected count may be used without SUV and/or activity concentrationcalculation. For either quantitative or qualitative SPECT, the correctedcounts are used to reconstruct the emissions as a function of location.The relative amounts of emissions from different locations arereconstructed.

The reconstructed emission distribution is imaged. Any imaging may beused, such as extracting a planar representation from voxelsrepresenting the distribution. A multi-planar reconstruction may begenerated. In one example, a three-dimensional rendering usingprojection or surface rendering is performed. The resultingthree-dimensional representation is displayed on the two-dimensionalscreen.

While the invention has been described above by reference to variousembodiments, it should be understood that many changes and modificationscan be made without departing from the scope of the invention. It istherefore intended that the foregoing detailed description be regardedas illustrative rather than limiting, and that it be understood that itis the following claims, including all equivalents, that are intended todefine the spirit and scope of this invention.

I (we) claim:
 1. A method for dead time determination for a gamma camera(18), the method comprising: detecting (34), with the gamma camera (18),a count rate from a radioisotope source (26) connected adjacent to thegamma camera (18); detecting (36), with the gamma camera (18), emissionsfrom within a patient (22) while detecting the count rate; determining(40) the dead time from the count rate; and correcting (42) a count ofthe emissions as a function of the dead time.
 2. The method of claim 1wherein detecting (34) the count rate comprises detecting (34) with theradioisotope source (26) being a shielded source (26) connected to emitat a corner of the gamma camera (18).
 3. The method of claim 1 whereindetecting (34) the count rate comprises detecting (34) in response to aradioisotope of the radioisotope source (26), the radioisotope having agreater half-life by a factor of at least ten than a radiotracergenerating (46) the emissions from within the patient (22).
 4. Themethod of claim 1 wherein detecting (36) the emissions comprisesoperating the gamma camera (18) in a fully integrated mode.
 5. Themethod of claim 1 wherein determining (40) the dead time from the countrate comprises calculating (44) a ratio of the count rate to a rate fromwith no system dead time.
 6. The method of claim 5 wherein correcting(42) the count comprises dividing the count by the ratio.
 7. The methodof claim 1 further comprising: generating (46) a single photon emissioncomputed tomograph image as a function of the corrected count.
 8. Themethod of claim 1 further comprising: calculating (44) an activityconcentration as a function of the corrected count.
 9. A single photonemission computed tomography (SPECT) system comprising: a gamma camera(18); a shielded point source (26) connected to emit radiation at thegamma camera (18); detection electronics (20) configured to detectemissions, including the radiation from the shielded point source (26)and radioisotope emissions from a patient (22); and a processor (12)configured to correct for dead time of the detection electronics (20),the correction being a function of real-time detection of the radiationfrom the shielded point source (26).
 10. The SPECT system of claim 9wherein the shielded point source (26) connects to the gamma camera (18)with a hole in a shield directed to the gamma camera (18).
 11. The SPECTsystem of claim 9 wherein the shielded point source (26) comprises along-lived source (26) of the radiation relative to the radioisotope.12. The SPECT system of claim 9 wherein the detection electronics (20)are configured to detect the emissions without pile-up separation. 13.The SPECT system of claim 9 wherein the detection electronics (20) areconfigured to detect the radiation from the shielded point source (26)with an energy window for a range of energies different than for theradioisotope emissions.
 14. The SPECT system of claim 9 wherein thedetection electronics (20) are configured to detect the emissions,including the radiation and the radioisotope emissions, during ascanning session for a patient (22).
 15. The SPECT system of claim 9wherein the processor (12) is configured to calculate a ratio of a firstnumber of the emissions of the radiation to a second number from aperiod during which the radioisotope emissions do not occur.
 16. TheSPECT system of claim 9 wherein the processor (12) is configured toweight a count of the radioisotope emissions as a function of a numberof the emissions of the radiation as the correction.
 17. A method fordead time determination for an emission detector, the method comprising:detecting (32), with a detector, first emissions from a patient (22) andsecond emissions from a point source (26), the second emissionssubjected to dead time from the detection of the first emissions; andcorrecting (38), by a processor (12), a count of the first emissions asa function of a count of the second emissions.
 18. The method of claim17 wherein detecting comprises detecting the first and second emissionsin real-time during a patient (22) scan; further comprising detecting(30) third emissions not during the patient (22) scan and not subjectedto the dead time; wherein correcting (42) comprises correcting (42) as afunction of the count of the second emissions and a count of the thirdemissions.
 19. The method of claim 17 wherein detecting (32) comprisesdetecting (32) in a fully integrated mode of a single photon emissioncomputed tomography (SPECT) system.
 20. The method of claim 17 whereindetecting (32) comprises detecting (36) the first emissions in a firstenergy range and detecting (34) the second emissions in a second energyrange different than the first energy range.